---> dang Diggs beat me to it
When I installed my mounts I used a 3/4" piece of plywood to spread the load on top of my jack's cradle. Dimension of the plywood are approx 18"x18". This works fine as long as you don't go crazy!
I use these same plywood pieces under my jack stands as well to disburse their load and help the stands not dig into the pavement.
The trick is just to be slow and cautious when doing this you don't want to apply any more pressure than is really needed. Oh and make sure your car is on a level surface otherwise you're just asking for trouble.